The Colors of Exobiology

by Paul Gilster on June 22, 2007

Speaking of bio-signatures, as we did at the end of yesterday’s post on planetary atmospheres, take note of the Virtual Planet Laboratory, a working group at the Jet Propulsion Laboratory that is trying to figure out what life’s markers might look like across a wide range of biological types. The most obvious signature for life itself is the presence of unusual combinations of things. A world without life shouldn’t, for example, give us strong signatures in both methane and oxygen simultaneously.

We looked at this subject in an April post, but a recent news release prompts me to put it back into play. The work is highly theoretical, proceeding as it does with no current examples of extrasolar planetary spectra from terrestrial-class worlds to look at. But we can begin with photosynthesis and its variants, as discussed here by Robert Blankenship (Washington University, St. Louis), a member of the group of researchers:

“When you consider another world you’ve got to find that life there depends on photosynthesis in the broad sense, but it’s probably not identical to the way that photosynthesis works here. You’ll need molecules that absorb light that are highly colored, but whether they have the same green colors we know on Earth is unlikely.”

Plants that are black instead of green? Blankenship thinks they’re possible. It’s wonderful to think about what Chesley Bonestell might have come up with to illustrate such concepts, but Doug Cummings’ work below handles odd foliage colors nicely. And Blankenship points out that chlorophyll isn’t as efficient as it might be in harnessing light’s energies. A black molecule could absorb all the ambient light, depending on the spectrum of light impinging on the planetary surface. The size and intensity of the star in question obviously become key players here.

“Apparently the vegetable kingdom in Mars, instead of having green for a dominant colour, is of a vivid blood-red tint,” wrote H.G. Wells in The War of the Worlds, an 1898 novel whose landscape speculations now can be extended to other solar systems, Mars having failed in the vegetation department. In a recent paper, the researchers speculate that among the best candidates for the study of biomarkers are M-class stars, red dwarfs like Barnard’s Star, Proxima Centauri and the widely touted Gliese 581:

Biosignatures — both atmospheric and surface — on planets around M stars may actually be easier to detect than those around F, G, or K stars. The modeled atmospheres of M star planets of Segura, et al. (2005) reveal that low UV radiation from quiescent M stars could result in higher concentrations of biogenic gases CH4, N2O, and CH3Cl. Tinetti, et al. (2006) found the “red edge,” shifted to the NIR, to be easier to detect through modeled clouds than the plant red edge. These prospects to detect life should motivate continued investigations into M star atmospheres and the spectral adaptations of extrasolar photosynthesis.

The ‘red edge’ in question can be helpful because as you push into the near-infrared (NIR), chlorophyll becomes much more reflective (this begins at wavelengths in the area of 700 nm and greater). Thus foliage can throw an interesting signature at these wavelengths, telling us that there is a method of light capture and energy storage at work on a distant world. The same paper cited below also goes into M star flare activity and its effects on plants pushing into deeper water for flare protection.

The paper, of which Blankenship is a co-author, is Kiang et al., “Spectral signatures of photosynthesis II: coevolution with other stars and the atmosphere on extrasolar worlds,” Astrobiology 7 (2007), pp. 252-274 (abstract). See also Kiang et al., “Spectral signatures of photosynthesis I: Review of Earth organisms,” Astrobiology 7 (2007), pp. 222-251 (abstract).

1) our conception that photosynthetic organisms are green ‘reflects’ a bias towards terrestrial plants. most photosynthetic organisms on earth are red (marine algae). several others are golden, and there are also brown and blue algae. the colors are due to secondary pigments.

2) the first photosynthetic organisms on earth were purple (rhodobacteria, which still exist). their initial predominance in the seas may have selected for green (anti-purple) competitors. (note1: green is exactly the opposite of the color that photosynthetic organisms ‘should’ have on earth: they REFLECT the strongest solar wavelengths.) (note 2: the rod cells in our eyes use much the same system that rhodobacteria use for photosynthesis; in our case, they simply ‘detect’ light.)

3) photosynthesis came late. it’s not a simple system. chemo-autotrophs came first. many still exist.

4) equating life with photosynthesis is shortsighted (terrocentric?). life exploits a loophole in the second law of thermodynamics (2LD): 2LD states that the entropy of a closed system must always increase… on average. living organisms get around this by accelerating the ‘flow’ of entropy around them and harvesting a portion of the difference between natural flow and accelerated flow to fuel local decrease in entropy. (net entropy increases.)

5) point 4 must be interpreted in the context of the first law of thermodynamics: no transfer of energy is 100% efficient. (stay with me!)

6) therefore, the presence of life accelerates the decomposition of (increase in entropy of) organic (and inorganic — in the case of chemo-autotrophs) molecules. (consider a sterile corpse on a lifeless planet.)

7) detecting life devolves ‘simply’ to determining how fast organic molecules decay, compared to the expected rate. since decomposition releases heat, this comparison could be done by calculating the energy balance of a lifeless planet, calculating the expected temperature (physics so simple that i could do it) and comparing observed to expected. observed > expected = life.

8) “did i ever tell you i wish my name was robert? that way, i could say, “yes, that’s my name: robert, robert blankenship. oh, and did i say i wish my last name was blankenship?”
-jack handey (except, he said “todd.”)

Several well-known artists are represented in the link that James sent. In terms of depicting foreign worlds, another terrific artist would be Bill Hartmann. His Cycles of Fire are among my all-time favorites.

Something about that artist’s impression bugs me (apart from it being a really blatant “I can play with Photoshop filters”…): on an alien world the sky would not necessarily be blue! The star’s spectrum, the thickness and composition of the atmosphere, etc. would all have an effect.

However, I do think the point of colors is a very good one. There are plenty of plants that don’t have green leaves. The question of efficiency is an important one. If the wavelength of light is changed, then the transfer of energy (exciting an electron if I recall correctly) changes. I don’t know the details anymore (I may have once…). However, I can imagine a situation arising where a particular wavelength of light is much more efficient for photosynthesis or photosynthetic-like processes. Although in some organisms a less-efficient system might be compensated with other advantageous features, in general the more efficient process would prevail.

If someone is an expert on photo system I and II, I’d love a recap of how the work… or maybe I should just go and check wikipedia.
-Zen Blade

Let’s remember that we did not even know how photosynthethis works until possibly recently when a proposed quantum computation method was postulated before we speculate on how alien plant systems work. The more we learn, the more we (should) realize the depths of our ignorance about this universe. It’s going to be very difficult to detect functioning biospheres across interstellar distances. In the interim, I think that step #7 proposed several posts ago is our best operating bet.

I think that in the long run photosynthetic life will prevail and dominate at the surface of a planet (I mean rather than other autotrophic mechanisms), because it makes use of the most abundantly available energy source there, which in turn favors an oxygen rich atmosphere.
However, the question is: since photosynthesis is such a complex process, how long did it take on earth, after life emerged, before photosynthetic life dominated and the earth atmosphere became O2 rich? I assume that something is known about this.

Abstract: The detection of exolife is one of the goals of very ambitious future space missions that aim to take direct images of Earth-like planets. While associations of simple molecules present in the planet’s atmosphere (O2, O3, CO2, etc.) have been identified as possible global biomarkers, we review here the detectability of a signature of life from the planet’s surface, i.e. the green vegetation.

The vegetation reflectance has indeed a specific spectrum, with a sharp edge around 700 nm, known as the “Vegetation Red Edge” (VRE). Moreover vegetation covers a large surface of emerged lands, from tropical evergreen forest to shrub tundra. Thus considering it as a potential global biomarker is relevant.

Earthshine allows to observe the Earth as a distant planet, i.e. without spatial resolution. Since 2001, Earthshine observations have been used by several authors to test and quantify the detectability of the VRE in the Earth spectrum.

The vegetation spectral signature is detected as a small ‘positive shift’ of a few percents above the continuum, starting at 700 nm. This signature appears in most spectra, and its strength is correlated with the Earth’s phase (visible land versus visible ocean). The observations show that detecting the VRE on Earth requires a photometric relative accuracy of 1% or better.

Detecting something equivalent on an Earth-like planet will therefore remain challenging, moreover considering the possibility of mineral artifacts and the question of ‘red edge’ universality in the Universe.

Comments: Invited talk in “Strategies for Life Detection” (ISSI Bern, 24-28 April 2006) to appear in a hardcopy volume of the ISSI Space Science Series, Eds, J. Bada et al., and also in an issue of Space Science Reviews. 13 pages, 8 figures, 1 table

Abstract: In a famous paper, Sagan et al. (1993) analyzed a spectrum of the Earth taken by the Galileo probe, searching for signatures of life. They concluded that the large amount of O2 and the simultaneous presence of CH4 traces are strongly suggestive of biology. The detection of a widespread red-absorbing pigment with no likely mineral origin supports the hypothesis of biophotosynthesis. The search for signs of life on possibly very different planets implies that we need to gather as much information as possible in order to understand how the observed atmosphere physically and chemically works.

The Earth-Sun intensity ratio is about 10^{-7} in the thermal infrared (10 micrometer), and about 10^{-10} in the visible (0.5 micrometer). The interferometric systems suggested for Darwin and the Terrestrial Planet Finder Interferometer (TPF-I) mission operates in the mid-IR (5 – 20 micrometer), the coronagraph suggested for Terrestrial Planet Finder Coronagraph (TPF-C) in the visible (0.5 – 1 micrometer). For the former it is thus the thermal emission emanating from the planet that is detected and analyzed while for the later the reflected stellar flux is measured. The spectrum of the planet can contain signatures of atmospheric species that are important for habitability, like CO2 and H2O, or result from biological activity (O2, O3, CH4, and N2O). Both spectral regions contain atmospheric bio-indicators. The presence or absence of these spectral features will indicate similarities or differences with the atmospheres of terrestrial planets and are discussed in detail and set into context with the physical characteristics of a planet in this chapter.

We’ve already found over 250 extrasolar planets, and more are continuing to be discovered fairly often. With all of these new planets popping up, the obvious question must be asked: how do we go about detecting whether or not they contain life? Though we can’t yet see features on the surface with even the most powerful of telescopes – and probably won’t be able to do so for a very long time – an analysis of the light coming from the planet may reveal if it is covered with life in the form of plants.

Dr. Luc Arnold of the CNRS Observatoire de Haute-Provence in France suggests that a spectral analysis of the light reflected off of a planet could determine whether or not it is covered with vegetation.

“All conceivable life forms, whether earthly or extraterrestrial,
require an energy source, and scientists are increasingly
employing a “Follow the Energy” approach in the search
for signs of habitability and life beyond Earth, as described
in a report in the December 2007 Special Issue (Volume 7,
Number 6) of Astrobiology, a peer-reviewed journal published
by Mary Ann Liebert, Inc.”

I know this article is a little dated, but as someone of science with art interests, I can’t help but note the quality work of the alien landscape. I had one similar from a news article about extrasolar worlds before by computer reformated >.< Is there a site or way to find a collection of alien landscapes (quality renderings)? Thanks

Andrew, maybe some of the readers will know of a good site for alien landscapes. I tend to find them scattered, often embedded in press materials from various universities or science facilities. Do keep an eye on talented artists like Lynette Cook, though:

Abstract: The so-called Vegetation Red-Edge (VRE), a sharp increase in the reflectance around $700 nm$, is a characteristic of vegetation spectra, and can therefore be used as a biomarker if it can be detected in an unresolved extrasolar Earth-like planet integrated reflectance spectrum.

Here we investigate the potential for detection of vegetation spectra during the last Quaternary climatic extrema, the Last Glacial Maximum (LGM) and the Holocene optimum, for which past climatic simulations have been made.

By testing the VRE detectability during these extrema when Earth’s climate and biomes maps were different from today, we are able to test the vegetation detectability on a terrestrial planet different from our modern Earth.

Data from the Biome3.5 model have been associated to visible GOME spectra for each biome and cloud cover to derive Earth’s integrated spectra for given Earth phases and observer positions. The VRE is then measured. Results show that the vegetation remains detectable during the last climatic extrema.

Compared to current Earth, the Holocene optimum with a greener Sahara slightly increases the mean VRE on one hand, while on the other hand, the large ice cap over the northern Hemisphere during the LGM decreases vegetation detectability.

We finally discuss the detectability of the VRE in the context of recently proposed space missions.

On a world that spins around two dim suns, the vegetation may well look black to human eyes.

By Alan Boyle

Researchers suggest that vegetation on an alien planet like Tatooine in “Star Wars” might well look black or gray to human eyes. But they probably wouldn’t seem devoid of color to the eyes of the aliens — assuming they have eyes, that is.

The conjecture comes from a paper presented by the University of St. Andrews’ Jack O’Malley-James at the Royal Astronomical Society’s National Astronomy Meeting in Wales. O’Malley-James is working on a Ph.D. project to assess the potential for photosynthetic life in multiple-star systems with different combinations of sunlike stars and red dwarfs.

Charter

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last nine years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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